How to get YearMonth field from a timestamp

Hi I have a normal time field with date and time. i use

date(floor([Execution Date]),'MMM-YYYY') AS yearmonth

to get the yearmonth. but i still can't aggregate based on that. because I notice in the list box, they show multiple same yearmonth, so the date is still there. how to get rid of date from my yearmonth so that i can calculate based on that. see the pic

I guess it is a simple question... maybe it is my way of search but i have not find answers in the forum.